The introduction of new electrical topologies in grid-connected wind turbines and the use of new control methods has made it possible to increase the integration of electricity generation sources from wind energy in the grid, thereby contributing to the proper functioning thereof. Both in the case of configurations where the electricity generator is fully disconnected from the grid due to the use of AC/DC/AC converters in the connection of the stator thereof to the grid and in the case of doubly-fed induction generators (DFIG), the rotor of which is fed by a converter of a reduced percentage of total wind turbine power, there are various techniques for fulfilling grid requirements and for guaranteeing a certain capacity for supporting electrical disturbances without disconnecting these systems from the grid. For example, both DFIG converters and full power converters having electric torque controlled by means of power electronics equipment allow independent control of the active power and reactive power generated, thereby contributing to controlling grid voltage.
During a voltage dip wherein the grid voltage is lower than the nominal voltage to a certain degree, a wind turbine should increase its in-phase electric current when the voltage is reduced in order to produce the same power. In certain circumstances, such as for example when the wind turbine is operating at near-nominal production levels, during deep voltage dips the limitation of current in the electrical components makes it unfeasible to maintain the same power levels.
Therefore, the appearance of a voltage dip causes acceleration of the rotor. Said dynamics produce an overload in different wind turbine components. Thus, for example, the sudden loss of electric torque during a voltage dip and the fast action of the pitch control system to brake the acceleration of the rotor exerts strong loads against the tower which can shorten the life of the wind turbine.
However, it must be noted that the sudden actuation of the pitch system is harmful to the tower on suddenly braking wind thrust on the rotor and on exciting certain vibration modes thereof. Additionally, the variation in torque caused by said reduction in power evacuated by the generator also causes sideways oscillation of the nacelle, corresponding to very lightly damped modes.
In the state of the art prior to this invention, in the event of a dramatic reduction in available power, the control system acts on the pitch until the position of the blades is such that the power captured by the rotor equals the power evacuable by the grid. Said action is performed at the maximum pitch rate allowed by the actuator. Controlling the blade at the maximum pitch rate of the actuator allows the maximum braking power to be exerted on the rotor by means of aerodynamics and prevents wind turbine stop, which would lead to disconnection from the grid. This enables compliance with grid connection regulations. There are also references, as described herein, of inventions aimed at reducing the oscillations of the gondola in the event of a voltage dip.
In the current state of the art, solutions such as that provided by Patent ES2333393B1, which discloses a wind turbine control method whereby, on detecting the voltage dip in the grid, the power generated by the turbine is reduced by acting on the pitch angle to a certain degree in accordance with the power available during a voltage dip and from the wind at any given time, and power generated by the electricity generator is reduced by reducing the torque. This control method equates the power generated in dip conditions to the power available to the grid in those conditions. Said power evacuable to the grid depends on the grid voltage level, reactive current level and the maximum current limits of the system.